Risk factors associated with preterm (<37+0 weeks) and early preterm birth (<32+0 weeks): univariate and multivariate analysis of 106 345 singleton births from the 1994 statewide perinatal survey of Bavaria - Martius, J A; Steck, T; Oehler, M K; Wulf, K H (Eur J Obstet Gynecol Reprod Biol, 1998)